Reductive Transformation of MnO2 Controls Thallium Remobilization: Differential Effects of Layered and Tunneled Structures

Mn oxides play a critical role in Tl scavenging and accumulation in the epibiotic environment. However, the effect of Mn oxide reduction in the Mn/Fe cycle on Tl mobilization is not clear. Herein, the influence of Mn oxide configuration, oxygen environment, and degree of reduction on MnO2 transformation and associated Tl species distribution is investigated. In oxic environments, both typical δ-MnO2 and α-MnO2 structures (i.e., layered and tunneled, respectively) can immobilize Tl(I) for a long time. In mild-to-moderate reducing anoxic environments, the drastic reductive transformation of δ-MnO2 results in Tl binding, mainly in an exchangeable form. In highly reducing environments, δ-MnO2 or α-MnO2 is converted to Manganite, resulting in the release of more Tl. Tl-LIII edge X-ray absorption spectroscopy indicates that oxidized Tl(III) (54-62%) is converted to structural Tl(I) (67-80%) and bound to interlayer/tunnel centers during the reductive transformation of MnO2, which enhances Tl exchange in δ-MnO2 and leads to Tl immobilization in α-MnO2. Our results show that anoxic Tl(I)-/Mn(II)-/Fe(II)-induced MnO2 transformation can enhance Tl mobilization, and tunneled MnO2 may have a more sustainable Tl immobilization potential than layered MnO2, which improves the general understanding of the geochemical behavior of Tl in different Mn-related reducing environments.